Effective Droplet Drying

ABSTRACT

In one aspect, the invention provides a system and method for spray drying a fluid. The system comprises a fluid reservoir ( 2 ), a spraying device ( 3 ) that comprises at least one outflow opening ( 4 ) for projecting droplets of fluid ( 5 ) from the reservoir ( 2 ) out of the at least one outflow opening ( 4 ) and at least one energy source ( 6 ) for at least partially drying the droplets. The spraying device ( 3 ) is arranged to project the droplets into a determinable droplet trajectory and the at least one energy source ( 6 ) is arranged for providing energy focused substantially on the droplet trajectory. It is an object of the invention to provide a system and method for reducing the amount of energy required for spray drying.

The invention relates to the field of drying fluids.

Drying a fluid means that droplets of the fluid are dried to becomeparticles. In the remainder, the terms particles and droplets generallyidentify the same entity, depending on a drying stage. Drying of fluidsis e.g. applied in the food industry for drying a nutrient or aningredient therefore, like dairy, proteins, carbon hydrates, fats, orcombinations thereof. Spray drying may further be applied for dryingfluids such as, but not limited to, detergents, pigments, catalysts,pharmaceuticals, cosmetics, polymeric resins, ceramic powders, powdercoating materials, adhesives, gypsum, cement, metal powders, etc.

In a conventional type of spray drying, the fluid is fed to a nozzle,which produces a mist of droplets of the fluid in a vessel. The dropletsare subsequently dried, e.g. in a stream of air in a drying tower. Adisadvantage of this method is that it takes a relatively large amountof energy for drying the droplets to become particles.

It is an object of the present invention to provide a system and methodfor reducing the amount of energy required for spray drying.

In one aspect, the invention provides a system for spray drying a fluid,the system comprising a fluid reservoir and a spraying device. Thespraying device comprises at least one outflow opening for projectingdroplets of fluid from the reservoir out of the at least one outflowopening and at least one energy source for at least partially drying thedroplets. The spraying device is arranged to project the droplets into adeterminable droplet trajectory and the at least one energy source isarranged for providing energy focused substantially on the droplettrajectory. By focusing, embodiments are meant to be encompassed whereinthe energy is confined in a limited space around the droplet trajectoryfor example, by a heated gas flow guide. By a ‘determinable trajectory’it is meant that a trajectory of individual droplets is largelydetermined by predefined course, for example a jet trajectory of jetteddroplets, or a flow trajectory of a carrier flow.

In another aspect, the invention provides a method for spray drying afluid. The method comprises the steps of projecting droplets of fluidfrom a reservoir out of at least one outflow opening and at leastpartially drying the droplets with energy provided by at least oneenergy source. The droplets are projected into a determinable droplettrajectory and the energy is focused substantially on the droplettrajectory.

FIGURES

FIG. 1 shows a schematic representation of the system for spray drying afluid according to the invention.

FIG. 2 shows a schematic representation of a second embodiment of asystem for spray drying a fluid according to the invention.

FIG. 3 shows a schematic representation of a third embodiment of asystem for spray drying a fluid according to the invention.

FIG. 4 shows a schematic representation of a fourth embodiment of asystem for spray drying a fluid according to the invention.

FIG. 5 shows an embodiment comprising a plurality of connected segments;

FIG. 6. shows an additional embodiment comprising tubular gas guides;

FIG. 7. shows an embodiment comprising a focused energy source accordingto the invention.

FIG. 8 shows embodiments of the focused energy source according to theinvention.

FIG. 9 shows embodiments of the focused energy source according to theinvention.

FIG. 10 shows embodiments of the focused energy source according to theinvention.

FIG. 11 shows an alternative embodiment including a segmented gas flowsystem.

FIG. 12 shows a detail of the segment interface in FIG. 11.

FIG. 13 shows a calculated design for a box formed gas flow guide.

FIG. 14 shows calculated flow profiles for the structure of FIG. 13.

DESCRIPTION

FIG. 1 shows an embodiment of the system for spray drying a fluidaccording to the invention. The system 1 comprises a reservoir 2 and aspraying device 3. The spraying device is in fluid communication withthe reservoir 2 for supplying the fluid to be spray dried to thespraying device. The spraying device comprises at least one outflowopening 4 and is arranged for projecting droplets of fluid 5 from thereservoir 2 out of the at least one outflow opening 4. An outflowopening may comprise a nozzle. The terms outflow opening and nozzle willbe used as synonyms in the following. A mechanism for projectingdroplets known from the prior art is the so-called Rayleigh break-upmechanism. A projecting apparatus based on this mechanism, typicallycomprises a vibrating nozzle or vibrating nozzle plate. The systemfurther comprises at least one energy source 6 for at least partiallydrying the droplets. The system is characterized in that the sprayingdevice is arranged to project the droplets into a determinable droplettrajectory and the at least one energy source 6 is arranged forproviding energy 7 focused substantially on the droplet trajectory.

In the embodiment shown in FIG. 1, the spraying device 3 comprises oneoutflow opening 4. It will be appreciated that it is also possible thatthe spraying device 3 comprises a plurality of outflow openings 4 forprojecting the fluid out of the plurality of outflow openings 4 forobtaining droplets of the fluid. Thereto, the outflow openings 4 maye.g. be arranged as bores in a surface of the spraying device 3.

To provide a determinable droplet trajectory, the embodiment of thespraying device 200 illustrated in FIG. 2A comprises feed pressuregenerating means 8 for providing the fluid to the outflow opening 4 at apredetermined feed pressure for obtaining a fluid jet. Other mechanismsfor providing a determinable droplet trajectory may also be used. Here,the feed pressure generating means 8 are arranged for keeping the feedpressure substantially constant. Hence, a fluid jet is generated, whilethe feed pressure generating means 8 substantially do not disturb thejet. Adding feed pressure generating means 8 is only one way to providea determinable droplet trajectory. Other ways are possible. In thisembodiment, the feed pressure generating means comprise a pump 9 forsupplying the fluid at the desired pressure.

In another embodiment of the spraying device according to the presentinvention 201, shown in FIG. 2B, the pump 9 is positioned between thereservoir 2 and the spraying device 3 such that the fluid from thereservoir 2 is supplied to the spraying device 3 under pressure. In thisembodiment, the pump 9 is arranged for keeping the feed pressuresubstantially constant. Thereto, the pump 9 may comprise a pressureregulator, such as an overpressure valve and/or a pressure regulatingvalve and/or a damper. Alternatively, or additionally, the feed pressuregenerating means 8 may be arranged for applying pressure to the fluidhydraulically or pneumatically for keeping the feed pressuresubstantially constant, i.e. the feed pressure generating means 8 mayuse pressurized liquid or gas, respectively, to exert a substantiallyconstant pressure on the fluid. In this embodiment the pump may bearranged to vary the flow rate so as to keep the pressure constant.

In another embodiment, the pump may be arranged so as to keep the flowrate constant, which results in a certain pressure. An example of such apump is a constant flow pump.

Another embodiment of the system for spray drying a fluid 300 is shownin FIG. 3. This embodiment is essentially the same as the previousembodiment. However, the spraying device 3 comprising a vibrating nozzle10 arranged for providing monodisperse droplets by Rayleigh break-up.Therefore this embodiment further comprises pressure varying means 10for varying the pressure of the fluid upstream of the outflow opening 4.When the jet is projected from the outflow opening 4, variations in thepressure of the fluid of the jet cause the jet to contract at points ofminimum pressure. Subsequently, the fluid jet will break up at thecontractions, thus forming droplets of fluid 5. The amplitude of thevariations in the pressure of the fluid may be approximately 10% of thefeed pressure. The amplitude of the variations in the pressure may be 1mbar to 100 bar, preferably smaller than or equal to 25 bar.

In the embodiment illustrated in FIG. 3, the pressure varying means 10comprise a control element 11, which is movable in a direction from/tothe outflow opening 4. Vibrating the control element 11 relative to theoutflow opening 4 causes the pressure of the fluid to vary between thecontrol element 11 and the outflow opening 4. When the fluid isprojected from the outflow opening 4 in the fluid jet, the variations inpressure in the fluid extend into the fluid jet. The pressure varyingmeans 10 may e.g. comprise a piezo-electric element, an electrostrictiveelement, an acoustic element, an electromagnetic actuator, a voice-coil,and/or mechanical means for moving the control element 11 in thedirection to/from the outflow opening 4.

If the spraying device 3 comprises a plurality of outflow openings 4, asingle control element 11 may be used for varying the pressure ofsubstantially all fluid jets generated by the plurality of outflowopenings 4.

In another embodiment similar to the embodiment shown in FIG. 3, thecontrol element is arranged for varying the pressure of the fluidupstream of the outflow opening 4 at a predetermined frequency.Preferably, the predetermined frequency is substantially constant.Hence, the fluid jet will contract, and subsequently break up, atsubstantially equidistant positions along the fluid jet. Thus,substantially equally sized droplets will be formed. Droplets may beprovided with a relatively narrow droplet size distribution, e.g. adroplet size distribution with a monodispersity index of less than 1,preferably smaller than 0.7, more preferably smaller than 0.1. Themonodispersity index of the droplet size distribution is defined as(d90-d10/d50), wherein d10, d50 and d90 represent the 10%, 50% and 90%droplet size percentiles, respectively.

When the fluid is substantially homogeneous, substantially equal sizeddroplets will have substantially equal mass.

This is only one possible way of providing monodisperse droplets, otherways may be possible.

In case of a plurality of outflow openings 4, the outflow openings 4 mayhave substantially identical dimensions, to allow droplets ofsubstantially identical dimensions to be ejected from each separateoutflow opening 4, hence allowing the droplets to be produced with therelatively narrow size distribution.

An advantage of providing monodisperse droplets may be that the amountof drying energy can be related to the size of the droplets and thedesired degree of drying. Because the droplets have substantially thesame size, the drying result of the droplets will be substantially thesame. This may reduce the amount of energy required for drying thedroplets because it is no longer necessary to choose the amount ofenergy such that even the largest droplets will reach the preferreddegree of drying. Also, because the droplets substantially have the samesize, formation of particles may be prevented that are smaller than thedesired droplet size. Smaller droplets could become too dry and evenburn. Although the burning of droplets could be useful in somesituations, it is typically unwanted. In contrast with conventionalatomizing systems, so called fines (ultra fine particles) are notformed, which is beneficial for efficient reuse of energy since finescan cause problems and pollution in heat exchanger systems.

Preferably, the amount of energy will be such that the droplets willcompletely and efficiently dry without forming a hard crust around thesurface of the droplet. Such a crust may prevent water in the inside ofthe droplet to diffuse to the surface and may therefore disturb thedrying process. There may however also be situations in which it isdesired to only partially dry the droplets. In yet other circumstances,it may be preferred to provide an amount of energy such that pyrolysisof droplets comprising an organometallic compound occurs. This may e.g.be useful in the field of droplet printing electrical components likeinsulators and semiconductors on substrates.

The present invention wherein monodisperse droplets are dried over apredeterminable trajectory further enables to optimize the dryingprocess by selectively providing energy along the trajectory. This meansthat the amount of energy supplied to the droplet may vary depending onthe position of the droplet in the trajectory and therefore depending onthe drying degree of the droplet. Examples of energy sources arrangedfor selectively providing energy along the droplet trajectory are shownin FIG. 8 and are described later.

Coagulation of droplets may be harmful for a narrow dropletdistribution. Several precautions may be taken to prevent coagulation.

First, the predetermined pressure varying frequency of the pressurevarying means 10 may be chosen such that a distance between two dropletsconsecutively ejected at the outflow opening 4 is greater than or equalto two times the droplet diameter, preferably greater than or equal tothree times the droplet diameter.

Second, in case of a plurality of outflow openings 4, the nozzles 4 maybe arranged for generating a plurality of mutually divergent jets of thefluid. When the jets are mutually divergent, the risk of coagulation ofdroplets ejected by mutually different nozzles 4 is reduced, since thedistance between the droplets ejected by the mutually different nozzles4 increases during flight.

Third, the distance between two adjacent nozzles 4 of the plurality ofnozzles 4 may be larger than 1.5 times a transverse dimension, such as adiameter, of at least one of two adjacent nozzles 4, more preferablylarger than 2 times, most preferably larger than or equal to 2.5 timesthe transverse dimension. This also reduces the risk of coagulation ofdroplets.

Also, referring to the FIG. 4 embodiment, to further enhance thedeterminability of the droplet trajectory, the aerodynamic positioningmay comprise accelerating the droplets 5 to prevent coagulation. Thismay reduce the probability of droplet coagulation.

The system for spray drying a fluid according to the present inventionmay be used with a variety of droplet generating systems. The dropletsmay e.g. be generated continuously or non-continuously, e.g. on adrop-on-demand basis. Further, droplets of low viscosity fluids may begenerated, e.g. droplets of saline solutions with a viscosity of 2 mPa·sand also high viscosity droplets may be generated, e.g. with a viscosityof 250 mPa·s.

Spray drying high viscosity fluids is desired for example in the foodprocessing industry, e.g. for drying milk. In conventional milk drying,the milk may be atomized and dried in a drying tower. Milk comprises asubstantial amount of water and removing such an amount of water in adrying tower may not be very energy efficient. Using high viscosityprinting systems enables to first extract water from the milk in anenergy efficient way and subsequently dry the extracted high viscosityof milk droplets. An exemplary viscosity of extracted high viscositymilk droplets is 250 mPa·s. Drying extracted milk droplets may be muchmore energy efficient than the conventional milk drying process.

Projecting the droplets into a determinable droplet trajectory andarrange an energy source for providing energy focused substantially onthe droplet trajectory, as in the present invention, may even furtherincreases the energy efficiency of the drying process.

To print high viscosity fluids, in another embodiment the sprayingdevice 3 is further arranged to cause a pressure drop in the fluidacross the outflow opening 4 which is larger than 15 bar, so as toenable projecting droplets of fluid with a viscosity of at least 100mPa·s. Projecting droplets of high viscosity fluids enables to spraydray fluids with a larger concentration of dry matter. This means thatless fluid has to be removed while drying the droplets. This mayincrease the energy efficiency.

For printing high viscosity fluids, the pressure drop in the fluidacross the outflow opening 4 is preferably between 50 bar and 400 barand more preferably between 100 and 200 bar.

The viscosity of the fluid is higher than 10 mPa·s, preferably higherthan 25 mPa·s, more preferably higher than 50 mPa·s even more preferablyhigher than 100 mPa·s, and most preferably higher than 200 mPa·s,determined at the temperature which in use prevails in the outflowopening 4. The temperature of the material to be printed is preferablybetween −50 and 300° C. and more preferably between 40 and 100° C. Theshear rate as in use present in the outflow opening 4 is preferablybetween 1·10⁴ and 1·10⁶ s⁻⁴ and more preferably 5·10⁵ s⁻¹, using acapillary viscosity meter.

The smallest transverse dimension, such as a diameter, of the outflowopening 4 may be smaller than or equal to 150 micrometer, preferablysmaller than or equal to 100 micrometer, more preferably smaller than orequal to 80 micrometer and most preferably smaller than or equal to 60micrometer.

The combination of the pressure drop, high viscosity and dimension ofthe nozzle 4 provide that droplets of the high-viscosity fluid can beproduced having a desired size of preferably smaller than or equal to250 micrometer in average and more preferably smaller than or equal to100 micrometer in average.

Spray drying a high viscosity fluid desires the feed pressure in theinterval of 15-3000 bar and preferably in the interval of 15-600 bar.

Spray drying a high viscosity fluid desires the control element (11)being positioned at a predetermined distance between 2 and 1500micrometer and preferably between 15 and 500 micrometer to the outflowopening (4). This provides the advantage that a pressure variationexerted to the high-viscosity fluid is prevented from being damped bythe high-viscosity fluid to an extent that the jet projected from theoutflow opening 4 does not experience a pressure variation with largeenough amplitude to effectively break up into droplets.

In an inkjet printing system, the transverse dimension of the droplets,e.g. the diameter depends on the flow rate and the pressure varyingfrequency. E.g. an outflow opening with a flow rate of 2.4 ml/min and apressure varying frequency of 40.000 Hz obtains an average dropletdiameter (d50) of 124 micrometer. The same flow rate of 2.4 ml/min and amuch lower frequency of 500 Hz results in a droplet diameter of 535micrometer. It is noted that droplet diameter refers to the diameter ofthe droplet directly after leaving the outflow opening. The dimensionsof the dried powder particle may be smaller.

In principle, the droplet diameter does not depend on the diameter ofthe outflow opening. However, the diameter of the outflow opening mayinfluence the possible flow rates. It might e.g. not be possible tostably push as little as 0.2 ml/min through the same 80 micrometernozzle.

For a flow rate of 2.4 ml/min, the diameter of the nozzle is preferably80 micrometer, and a flow rate of 0.6 ml/min is preferable combined witha 30 micrometer nozzle. The latter flow rate of 0.6 ml/min combined witha pressure varying frequency of 40.000 Hz results in droplets with adiameter of 78 micrometer and combined with a frequency of 500 Hz indroplets with a diameter of 337 micrometer.

To further enhance the determinability of the droplet trajectory, thesystem for spray drying a fluid may comprise a gas flow guide arrangedfor manipulating a droplet guiding gas stream to aerodynamicallyposition droplets into the predeterminable droplet trajectory.Furthermore, the gas flow guide may form an effective means in itself toprovide energy focused substantially on the droplet trajectory. The gasstream may e.g. be a stream of air. If it is desired to preventoxidation of the droplets of fluid, nitrogen, argon or any othersuitable oxidation preventing gas may be used. In some situations, aslight oxidation of the droplets may be helpful. In these cases e.g. amixture of oxygen and argon could be used.

An example of such a gas guided drying system 400 is shown in FIG. 4.The drying system 400 comprises a spray head 3 comprising a controlelement 11. While many variations of the gas guide are possible, in thisembodiment, the droplets of fluid 5 are projected in a tube 12 in whicha gas stream 13 is provided through a gas supply opening 14. It isnoted, that although this embodiment can be usefully employed with thevibrating nozzle arrangement 3 disclosed herebelow, the spray head mayinclude other atomizing means, including embodiments wherein the droplettrajectory becomes determinable only after reception in the gas guide12.

In more detail, the gas flow guide comprises an inlet piece 123 and anoutlet piece 124 defining a generic flow direction P and a droplettrajectory T along oppositely arranged upstanding walls 120, 122, thewalls defining an elongated space 122 in between the upstanding walls120, 122 of limited width and having an axial direction aligned with thedroplet trajectory T; in use arranged to provide a gradiented laminargas flow in the gas guide 12 and having a gas flow velocity equal orlarger than the particle velocity, thereby defining the particletrajectory T. Here, flow direction P and particle direction T arealigned in the same direction.

The gas stream 13 is preferably laminar and has a parabolic profile dueto the tube form of the gas guide 12.

The gas stream has preferably the same velocity as the droplets of fluid5 or a larger velocity. The droplets follow the stream of gas, butlittle differences in the size and shape of the droplets may cause themto deviate from their trajectory. Using the gradiented velocity profile,in particular a parabolic profile, of the gas flow, the droplets 5 arestabilized by an aerodynamic lift effect that substantially forces thedroplets into a preferred trajectory through the tube 12, for example,along the centre axis of the tube. It is noted that if the droplets 5have a higher velocity than the gas flow, this effect may be reversedand the droplet 5 will then move away from the centre of the flow, whichmay be undesirable.

Aerodynamically correcting the droplets may also prevent droplets frombeing stuck in the spray drying system and thus polluting the system.

Optionally a diffuser 15 is provided for effecting a gradual influx ofthe gas. Another option is to provide a plurality of gas streams 13′ and13″ through a plurality of gas supply openings 14′ and 14″ for furtherstabilizing the system. The gas streams may be conditioned, inparticular, dried and/or heated. Outflow openings 140 are provided todivert the outflow gas steams 13, 13′ 13″.

As further shown in FIG. 5, energy reuse may be provided by heatexchanger and conditioner systems 50. Aerodynamic positioning can beused to “shoot” droplets towards the next heating zone. Heat exchanger50 removes “cold“-”wet” gas and replaces it with preheated “dry“-”warm”gas. Alternatively, conditioning may be provided with non-recirculatedgas, in a way similar as shown in FIG. 11.

In particular FIG. 5 shows a plurality of connected segments 12, 12′each provided with a particle inlet 123 and a particle outlet 124, andan air flow regulating structure 125 arranged to accelerate the dropletstowards each droplet outlet 124; and have the air flow diverted from thedroplet trajectory 5.

Such a flow regulating structure may be provided as a constriction 125accelerating the flow until a Stokes number >1 so that the particlescontinue their trajectory and no longer follow the air stream. DistanceA should be short enough so that the droplets keep a Stokes number >1 sothey keep following their own trajectory. Distance B should be takenlong enough the give the droplets 5 time to slow down to such an extendthat the flow in the tube 12 is larger then the droplet speed, resultingin droplet stabilization. If the distance is too short, the droplets 5may be destabilized by the airflow causing them to hit the wall of thetube 12. Accordingly, distance B functions as a deceleration structureprovided between subsequent segments 12, 12′ to decelerate the dropletprior to receiving the particle in a particle inlet 123′ of a subsequentsegment 12′.

It is noted that other deceleration methods may be considered, such aslocally increasing a gas pressure or the like; or that a decelerationstructure may be dispensed with, as illustrated in the embodiments ofFIG. 11-14.

One aspect of the invention is an energy source 126 providing energyfocused substantially in the droplet trajectory. According to theembodiment, a heater 126 may be arranged in the walls 120 of the gasguide. It is noted that the gas guide 12 may, through properdimensioning, have any orientation respective to the gravity direction,but is preferably held substantially horizontal. In addition, the tubes12 may be curved to provide compact designs, for example, in the form ofa coil system. In addition, the tubes 12 may be clustered. It is notedthat by (thermal) conditioning of the gas flow the heater 126 may bedispensed with while providing localized evaporation energy to thedroplets resulting in droplet drying.

FIGS. 6A and 6B show additional examples of a tubular gas guide 60 witha constant airflow, and heating being applied externally along the gasguide walls 120 to focus energy along the droplet trajectory. Heatingmay be provided by a fluid counter flow 61 opposite to the gas flowpulling the droplets through the tube 60. While FIG. 6A shows heating bya heated counterflow 61, alternative heating is possible, for example,by a heating coil wound (not shown) around the tube. Alternatively, FIG.6B shows heating the trajectory via an IR radiation 63 reflected byreflectors 64 towards the trajectory, in a way further exemplified inFIG. 7.

FIG. 7 illustrates an embodiment of an energy source 600 according tothe present invention, wherein the reflective energy focusing element 20comprises at least part of an elliptical mirror 17 and wherein the atleast one energy source 6 is positioned in a first focal point 16 of theat least part of an elliptical mirror 17 so as to focus at least part ofthe radiated energy in a second focal point 18.

In this embodiment, the energy source 600 is positioned in a first focalpoint 16 of at least part of an elliptical mirror 17 so as to focus atleast part of the radiated energy in a second focal point 18. Focusingthe energy may increase the drying efficiency and/or drying speed asdescribed above. In this embodiment, the energy source 600 in the firstfocal point 16 may e.g. comprise a heater with a glow spiral. However,other energy sources like infrared light sources are also possible. FIG.7A shows a top view of this embodiment of the energy source 600, whereinradiation beams 19, 19′, 19″, 19′″, 19″″, 19′″″ and 19″″″ are shown.(For clarity, in the following figures, not all the radiation beams 19are shown.) Radiation beam 19 travels directly from the energy source 6in the first focal point 16 to the second focal point 18. The otherbeams travel indirectly, i.e. being reflected by the mirror 17, from thefirst to the second focal point. FIG. 7B clearly shows a side view ofthis embodiment, wherein the radiation beams 19 travel from the energysource 6 in the first focal point 11 to focus energy 7 in the secondfocal point 18.

In conventional spray drying techniques, heated air is applied, e.g.convective or direct to the droplets. Air heating reduces air relativehumidity, which is the driving force for drying. Besides, highertemperatures speed up diffusion of water inside the solids, so drying isfaster. However, quality considerations limit the applicable rise to airtemperature. Too hot air may lead e.g. to crust formation or ‘casehardening’.

When applying radiation driven drying, e.g. dielectric drying, dropletsare heated and dried by means of radiofrequency or microwaves beingabsorbed inside the material. Radiation driven drying may be moreefficient and/or faster than air drying techniques. First, thewavelength of the radiation source, e.g. an infrared source may bematched to the absorption characteristics of the material. Otherradiation sources are possible, e.g. light sources in another than theinfrared spectrum. Second, it may be possible to apply more power to thedroplets via radiation than via heat drying without burning thedroplets, forming a crust or causing the occurrence of case hardeningeffects'. Preliminary experiments have shown that droplets may be spraydried eventually over a trajectory between 50 centimetres and one meter.This is very advantageous compared to the big drying towers in use forapplying conventional spray drying methods.

FIG. 8 shows several alternatives for configuring the focused energysource. FIG. 8A shows an energy source 601 comprising four parts ofelliptic mirrors 17 positioned orthogonally towards each other, suchthat the four second focal points 18 of the mirrors 17 overlap. Thisway, the energy of four energy sources 6 is collected in one sharedfocusing point. In FIG. 8B, an energy source 602 is shown, comprisingfive adjacent mirror structures, wherein each mirror structure comprisestwo elliptic mirrors 17 being positioned opposite each other, withoverlapping second focal points 18 wherein the energy of two energysources 6 is focused. This configuration is adapted to a spray systemcomprising five outflow openings 4 being positioned on a line.

FIG. 9 shows embodiments of focused energy sources wherein thereflective energy focusing element 20 is arranged to focus the radiatedenergy of the at least one energy source 6 in a plurality of focalpoints.

This is advantageous because preliminary experimental results show thatthe energy needed to dry a typical droplet train may be between 0.1 and10 Watt. The power of an infrared (IR) energy source 6 is typically atleast 1000 Watt, which may be too high and burn the droplets instead ofdrying them with the risk to cause an explosion.

In FIG. 9A, an energy source 603 is shown comprising an energy source 6being positioned within a reflective energy focusing element 21 thatfocuses the energy of the energy source 6 on seven beams of energy 7.

The energy source 604 shown in FIG. 9B is similar to the configurationof FIG. 8A, wherein the radiation of one energy source 6 is distributedover four beams of energy 7. The energy source 605 shown in FIG. 9C issimilar to the configuration of FIG. 8B, wherein the radiation of fiveenergy sources 6 is distributed over ten beams of energy 7.

An alternative way to distribute the energy of a single energy sourceover a plurality of droplet trains is to position a plurality of outflowopenings above a single beam of energy. This may e.g. be done in theconfiguration shown in FIG. 7A.

In the above it was discussed that the drying process may be optimizedby selectively supplying energy along the droplet trajectory. FIG. 10Ashows an energy source 606 comprising a plurality of energy sources 6,6′ and 6″ to provide the desired energy profile 7. Each energy source 6delivers the desired amount of energy 7. FIG. 10B shows an energy source607 with one energy source 6 that delivers the desired energy profile 7.The one energy source 6 may e.g. comprise a glow spiral designed in sucha way that it delivers the desired energy profile 7.

Other energy sources combining elements of the energy sources asdescribed above may be used without departing from the broader spiritand scope of the invention as set forth in the appended claims.

An alternative way to provide focused energy to the droplets may be toproject the droplets into streams of drying air. FIG. 11 shows anexample of such a stream of drying air.

In particular, FIG. 11A shows a side view of a segmented gas flow system110, each provided with a droplet inlet 111 and a droplet outlet 112,and an air flow regulating structure 120, 121 arranged to accelerate thedroplets towards each droplet outlet 112; and have the air flow divertedfrom the droplet trajectory 116.

The segmented gas guide structure of FIG. 11A is formed by a connectedseries of guides 115, 115′, each having a droplet inlet structure 111and a droplet outlet structure 112, that, in the shown example, isformed as structures 111, 112 at least partially separate from thecentral flow guide structure 113 providing generic flow P. The outletstructure 112 of the previous segment 115 is aligned with the inletstructure of a subsequent 115′ thus forming a segment interface 111′, sothat the particle travels in a substantial straight trajectory 116 frominlet to outlet structures via the segment interface 111′. The verticalextension of the planar flow region is indicated by reference numeral117. It is noted that the particle trajectory T is not aligned with thegeneral flow P.

FIG. 11B shows an enlarged axial front view of the gas guide 115 havingoppositely arranged upstanding walls 120, 121 converging in thedirection of gravity, to provide, in use, a substantially planar flow116 in a direction away from the generic flow direction P and having aflow velocity component oriented against the direction of gravity gbeing smaller than the flow velocity component of the generic flow; andbeing gradiented to provide droplet flotation. As can be seen, aneffective planar region 116 is formed in the gas guide of a flow havinga substantial planar form and a small vertical extension 117. In frontview, inlet zone 111, central flow guide structure 113 and outlet zone112 can be discerned.

FIG. 12 shows a detail of the segment interface 111′, where it can beseen that the inlet zone 111 has an inlet segment separation wall part1110 aligned with an outlet segment separation wall part 1120 of theoutlet zone 112 of a preceding segment. This arrangement prevents crossflow of an outlet flow P exiting outlet 112 entering the inlet 111 of asubsequent segment 115 and promotes proper droplet flow T.

FIGS. 13 and 14 show a further exemplification of such a gas guidestructure 115 in partial perspective view partially facing front shield131. In effect, a planar flow T is provided wherein the particle istransported along a trajectory 116 in a direction away from the genericflow profile 118 formed by the gas flow structure. FIG. 13 shows acalculated design for a box formed gas flow 115 guide having convergingsidewalls (see FIG. 11) in the direction of gravity. In addition,oppositely arranged bottom and top walls 119, 129 may define a wedgeshaped volume in between said walls converging in the direction ofgeneric flow P.

The flow provides the effect of an effective balancing height wheredroplets of various sizes and weights can be balanced to a substantiallyconstant flotation height. A wedge shaped boxed design 115 withconverging flow 118 may accelerate the gas flow in the gas guide, andenhance the forming of the planar flow profile 116 in a direction awayfrom the generic flow profile 118. Gas inlet zone 130 may have ashielding part 131 provided with a particle inlet opening. The inletshield 131 prevents cross flow between two connected segments byproviding an inlet flow zone 130 separate from the generic flow 118.This inlet flow zone 130 forms the beginning of the planar flow 116,extending towards an outlet zone shaped with a corresponding outlet zone132 protruding from the bottom wall 129.

FIG. 14 shows the calculated flow profiles for the wedge shapedstructure of FIG. 13. FIG. 14A shows an X-velocity profile, detailing asubstantially constant high forward velocity component in the planarflow, defining an X-direction along the trajectory 116 of a particle.The X-velocity component along the droplet trajectory is substantiallyhigher than the X-velocity of the generic flow; an illustrativedifference is 4.5 m/s of the planar flow against 3.5 m/s of the genericflow. The FIG. 14B diagram shows a corresponding decreased verticalcomponent of the planar flow defining a Y-velocity profile. Here it isshown that the local vertical flow velocity in the planar profile issubstantially lower than the generic flow component; an illustrativedifference, depending on geometry, is 0.45 m/s of the planar flowagainst 0.65 m/s of the generic flow.

It is noted that although in the above the invention is mainlypositioned in the field of spray drying liquids for providing powders,the invention may also be applied in other fields, e.g. the field ofdroplet printing electrical components like insulators andsemiconductors. In this particular field, metal salts are dissolved,preferably in water but also in organic solvents and printedsubsequently.

Providing energy along the droplet trajectory causes pyrolysis andmelting of an organometallic compound and allows a metal layer to beprinted on the surface of a substrate. To print ceramic elements,ceramic suspensions are printed, dried and sintered.

The detailed drawings, specific examples and particular formulationsgiven serve the purpose of illustration only. The embodiments describedshow vertical, line shaped trajectories. It is however also possible touse other, e.g. horizontal or curved trajectory forms. Furthermore,other substitutions, modifications, changes, and omissions may be madein the design, operating conditions, and arrangement of the exemplaryembodiments without departing from the scope of the invention asexpressed in the appended claims.

Unless physically impossible, any feature developed in the embodimentsis deemed to be disclosed in combination with any other feature of otherembodiments, specifically as elaborated in the subsequent claims.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. The word ‘comprising’ does notexclude the presence of other features or steps then those listed in aclaim. Furthermore, the words ‘a’ and ‘an’ shall not be construed aslimited to ‘only one’, but instead are used to mean ‘at least one’, anddo not exclude a plurality. The mere fact that certain measures arerecited in mutually different claims does not indicate that acombination of these measures cannot be used to advantage.

1-15. (canceled)
 16. System for spray drying a fluid, the systemcomprising: a fluid reservoir; a spraying device, the spraying devicecomprising at least one outflow opening, for projecting droplets offluid from the reservoir out of the at least one outflow opening; and atleast one energy source for at least partially drying the droplets,wherein the spraying device is arranged to project the droplets into adeterminable droplet trajectory and the at least one energy source isarranged for providing energy focused substantially on the droplettrajectory.
 17. The system according to claim 16, wherein the sprayingdevice comprises a vibrating nozzle arranged to provide monodispersedroplets by Rayleigh break-up.
 18. The system according to claim 16,wherein the energy source is further arranged for selectively providingenergy along the droplet trajectory.
 19. The system according to claim16, wherein the system further comprises a gas flow guide arranged formanipulating a droplet guiding gas stream to aerodynamically focusdroplets into the determinable droplet trajectory.
 20. The systemaccording to claim 19, wherein the gas flow guide is tubular.
 21. Thesystem according to claim 19, wherein the gas flow guide comprises aninlet piece and an outlet piece defining a generic flow direction and adroplet trajectory along oppositely arranged upstanding walls, the wallsdefining an elongated space of limited width and having an axialdirection aligned with the droplet trajectory; in use arranged toprovide a gradiented laminar gas flow in the gas guide and having a gasflow velocity equal or larger than the droplet velocity, therebydefining the droplet trajectory.
 22. The system according to claim 21,wherein the gas flow guide has a box form and wherein the oppositelyarranged upstanding walls are converging in the direction of gravity, toprovide, in use, a substantially planar flow in a direction away fromthe generic flow direction and having a flow velocity component orientedagainst the direction of gravity being smaller than the flow velocitycomponent of the generic flow; and being gradiented to provide dropletflotation.
 23. The system according to claim 19, wherein the gas flowguide further comprising oppositely arranged bottom and top wallsdefining a wedge shaped volume in between said walls converging in thedirection of flow.
 24. The system according to claim 19, wherein theenergy source is provided as a heater arranged to heat the walls of thegas guide.
 25. The system according to claim 19, wherein the energysource is provided as an air conditioner providing a conditioned airflow in the gas guide.
 26. The system according to claim 19, furthercomprising a plurality of connected segments, each provided with adroplet inlet and a droplet outlet, and an air flow regulating structurearranged to accelerate the droplets towards each droplet outlet; andhave the air flow diverted from the droplet trajectory.
 27. The systemaccording to claim 26, wherein between a droplet outlet and a dropletinlet a droplet deceleration structure is provided to decelerate thedroplet prior to receiving the particle in a particle inlet of asubsequent segment.
 28. The system according to claim 16, wherein the atleast one energy source is positioned with respect to a reflectiveenergy focusing element so as focus the energy from the at least oneenergy source in at least one focal point.
 29. A method for spray dryinga fluid, the method comprising the steps of projecting droplets of fluidfrom a reservoir out of at least one outflow opening; and at leastpartially drying the droplets with energy provided by at least oneenergy source, wherein the droplets are projected into a determinabledroplet trajectory and the energy is focused substantially on thedroplet trajectory.
 30. The method according to claim 29, wherein themethod further comprises the step of manipulating a droplet guiding gasstream to aerodynamically focus droplets into the determinable droplettrajectory.